Light pillar

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Nocturnal light pillars caused by light reflected through ice fog in Cambridge Bay, Nunavut, Canada

A light pillar is an atmospheric optical phenomenon in which a vertical beam of light appears to extend above and/or below a light source. The effect is created by the reflection of light from tiny ice crystals that are suspended in the atmosphere or that comprise high-altitude clouds (e.g. cirrostratus or cirrus clouds). If the light comes from the Sun (usually when it is near or even below the horizon), the phenomenon is called a sun pillar or solar pillar. Light pillars can also be caused by the Moon or terrestrial sources, such as streetlights.

Formation[edit]

Scheme of light pillars formation

Since they are caused by the interaction of light with ice crystals, light pillars belong to the family of halos. The crystals responsible for light pillars usually consist of flat, hexagonal plates, which tend to orient themselves more or less horizontally as they fall through the air. Their collective surfaces act as a giant mirror, which reflects the light source upwards and/or downwards into a virtual image. As the crystals are disturbed by turbulence, the angle of their surfaces deviates some degrees from the horizontal orientation, causing the reflection (i.e. the light pillar) to become elongated into a column. The larger the crystals, the more pronounced this effect becomes. More rarely, column-shaped crystals can cause light pillars as well.[1] In very cold weather, the ice crystals can be suspended near the ground, in which case they are referred to as diamond dust.[2]

Unlike a light beam, a light pillar is not physically located above or below the light source. Its appearance of a vertical line is an optical illusion, resulting from the collective reflection off the ice crystals, only those of which that appear to lie in a vertical line direct the light rays towards the observer (similar to the reflection of a light source in a body of water).[3]

Images[edit]

See also[edit]

References[edit]

  1. ^ "Sun pillars from column crystals". www.atoptics.co.uk.
  2. ^ "APOD: 2013 December 18 – Light Pillars over Finland". apod.nasa.gov.
  3. ^ "Light Pillars". www.atoptics.co.uk.

External links[edit]